This invention relates to controls for three phase induction motors and more particularly to a combined soft start and dynamic braking control for such a motor.
A three phase induction motor is operated by connecting a three phase power supply to the stator thereof to produce a rotating magnetic flux field which, in turn, causes the rotor of the motor to rotate.
It is well known in the art to connect the three phase power supply to the stator by means of either a full-wave or a half-wave voltage controller that incorporates silicon controlled rectifiers, and to control the firing angles of the rectifiers so as to gradually increase the supply of current to the stator. In this manner, a soft start is provided for the motor, i.e., the motor is brought up to speed in a gradual manner.
It is further well known in the art to dynamically brake a three phase motor, when the three phase power has been disconnected from the motor, by rectifying one phase of the supply voltage to provide a single pulsating D.C. current to the stator windings. This in turn induces a stationary magnetic flux field in the stator which brakes the cruising rotor of the motor.
In the prior art it has been necessary to provide separate control units to achieve the soft start and the dynamic braking action. These dual controls are not only expensive but also cumbersome because of the space taken up by such separate units which necessitate many electrical connections to them.